LONDON, Feb. 27 -- Author Dan Brown copied the central themes of his thriller "The Da Vinci Code" from a 1982 book, an attorney for two of that book's three authors argued in a London courtroom Monday.

But an attorney representing Brown's publisher, Random House, dismissed the claims as "scandalous" and "wild allegations, completely unsupported by facts" on the opening day of a copyright infringement trial involving one of the best-selling novels of all time.

The lawsuit potentially could involve millions of dollars in royalties and profits from "The Da Vinci Code," which has sold tens of millions of copies worldwide and has been made into a major Hollywood film, scheduled to open in May, starring Tom Hanks.

Brown, who generally shuns public appearances, sat in the front row of the small courtroom with neatly cropped blond hair, dressed in his trademark tan blazer, black turtleneck and khaki pants. Brown is not a defendant in the case, which is being brought against Random House, but he is expected to testify.

Both "The Da Vinci Code" and "The Holy Blood and the Holy Grail," also published by Random House, center on the idea that Jesus married Mary Magdalene, they had a child together, and the bloodline survives in secret to the present. Brown's 2003 book is a work of fiction; the earlier book, also an international bestseller, was presented as historical nonfiction.

Christian leaders have furiously rejected both books' central themes, and several books have been written to offer counter-arguments to theories advanced in Brown's book.

Lawyers who specialize in intellectual property law here said the case could clarify aspects of British copyright law. While it is clearly illegal to plagiarize from copyrighted material, it is less clear how much an author can use research and ideas presented in others' work, they said.

Brown has publicly acknowledged using "The Holy Blood and the Holy Grail" as part of his research in writing his book, but he has described it as a minor resource. John Baldwin, the attorney for Random House, argued in court that the book "did not have anything like the importance to Mr. Brown" asserted by authors Richard Leigh and Michael Baigent. The third author of "The Holy Blood and the Holy Grail," Henry Lincoln, is not a party to the lawsuit.

Their book is mentioned in "The Da Vinci Code." The name of one of Brown's key characters, Sir Leigh Teabing, combines Leigh's name and an anagram of Baigent's.

In his opening arguments, Jonathan Rayner James -- an attorney for Baigent and Leigh -- said Brown did not copy words from his clients' book but had taken "the idea you are left with when you've read the book."

James said the case was "not about stultifying creative endeavor" or "seeking a monopoly on creative ideas" or historical facts. He said it was about Brown appropriating the "architecture" and central points of his clients' work.

Baldwin, scheduled to make a formal opening statement Tuesday, told the judge during arguments Monday that the story told in "The Da Vinci Code" was markedly different from the earlier book. He said Baigent and Leigh had provided only "vague" arguments about how they believe Brown used the "architecture" of their book.

"It would be sweet if they told us," Baldwin said.

Random House Group CEO Gail Rebuck, in a written statement, said she was "saddened" by the case.

"Random House takes no pleasure in defending legal action that it believes is without merit," she said, "and we are confident that we shall prevail."

In August, Brown prevailed in a similar case filed against him in the United States, when a U.S. District Court judge in New York dismissed claims that he had infringed on the copyrights of two books by Lewis Perdue, "Daughter of God'' and "The Da Vinci Legacy.''

Speaking to reporters outside the courtroom Monday, Brown noted that, unlike his book, "The Holy Blood and the Holy Grail" theorizes that Christ did not die on the cross, as millions of Christians believe, but rather survived and later lived in France.

"Suggesting a married Jesus is one thing, but questioning the resurrection undermines the very heart of Christian belief,'' said Brown, who said he was raised as a Christian and sang in his church choir.

"The resurrection is perhaps the sole controversial Christian topic about which I would not desire to write," Brown said.